Change administrator password on windows 7 with autologon without knowing the old one

I have a Windows 7 machine that is set up with automatic logon. It is an internal machine not reachable by anything so security isn't an issue.

I don't remember what the password is, but in order to install some some software that will run as a service I need to input the current password.

The question is, how can I change the password on the administrator account without knowing the current password? Note that since it is set up with automatic logon, I have full access to the machine, but both Ctrl+Alt+Del change password and the control panel user manager change password requires the old password before changing.

Is this possible?